One known method of preparing an organic carbonates is by a direct procedure, which involves the oxidative reaction of an aromatic hydroxyl compound with carbon monoxide and oxygen in the presence of a noble metal catalyst and co-catalyst. The noble metal catalysts are the elements of group VIII (b) with palladium generally being the preferred noble metal used. The co-catalyst generally employed can be various manganese salts or cobalt salts in different oxidation states. In addition to these co-catalysts, a base, a quarternary ammonium salt and a desiccant can be used. A solvent may also be employed such as methylene chloride.
However, it has been determined that the removal of water from a reaction system in preparing a diaryl carbonate by carbonylations of alcohols leads to higher rates, higher selectively of the desired carbonate ester products and reduced degradation of catalytic species.
Techniques for the removal of water from organic carbonate synthesis from alcohols have been disclosed in the prior art. Several such processes include adsorption onto solid desiccants (U.S. Pat. No. 5,399,734 and EPO 0085347), distillation (JP04257546 and JP04261142) and stripping within the reactor with unreacted gaseous reactants or inert gaseous reactants (U.S. Pat. No. 5,498,742). However, these prior art processes have certain drawbacks. Adsorption onto solid desiccants such as molecular sieves is, by its very nature, a batch process in which the sieves must be periodically replaced or regenerated at high temperatures to provide for water removal capacity. This is economically undesirable in industrial practices.
The use of distillation, while potentially continuous, is undesirable due to the necessity of subjecting the reaction mixture to temperatures and pressures so that the water may vaporize.
Gaseous stripping within the reaction vessel, as previously disclosed, has disadvantages over this invention or several reasons: 1) stripping with inert components dilutes the reactant gases resulting in lower reactor productivity, 2) stripping with inerts is further limited to a 30% inert composition in the cited patent which is inferior to stripping with up to 100% inert gas in a separate vessel as per the disclosure in the present invention, 3) stripping within the reaction vessel limits the overall productivity of the system because optimal stripping and optimal reaction temperatures and pressures conditions do not coincide, 4) teachings in the cited patent do not describe use of a condensable stripping agent, thus requiring significant energy expenditure for recompression of non-condensable stripping gas to reaction pressure after water removal, and 5) absorbing or adsorbing or preferable condensing of water from the stripping gas in the cited patent is not as economic to the relatively low equipment and operating cost of the liquid-liquid separation of water as disclosed in this invention.